Fix a crash caused in the Incidence dtor. Fix a compile warning Use QString::clear() instead of assigned to a QString()

David, this fixes that crash I was seeing in Kontact I mentioned to you
a long time ago. I wonder if you can double-check the change in the
Incidence dtor.

CCMAIL: software@astrojar.org.uk

svn path=/trunk/KDE/kdepimlibs/; revision=828937